436 CHEMICAL ENGINEERING
calibrated for a reduction in electromotive force of 500/503. The error resulting from the parallel connection is thus 0.6 per cent or about 6°C. at 1,000°C. Hence, if the operator of the furnace switches couple number 5, for example, on the indicator when this couple is also connected to the recorder, both instruments will read about 6°C. low at 1,000°C. This error is usually insignificant.
A similar example will be considered for an indicator and recorder of low resistance. Let the line resistance r = 3 ohms as before and the resistances R of the indicator and of the recorder = 10 ohms each. The potential drop E across the terminals of either instrument bears the following relation to e the electromotive force of the couple.
Re _W ^~5T7~13e
When the two instruments are connected in parallel the potential drop across the indicator and recorder is as follows:
™ _ R _ 10
^ ~ R + 2r e ~ 16 &
The instruments are calibrated to read correctly when used separately, that is for a reduction in electromotive force by the factor 10/13. When used in parallel there is a reduction of electromotive force by the factor 10/16. The error resulting from a parallel connection is thus 19 per cent or about 190°C. at 1,000°C. Hence if the operator of the furnace switches couple number 5, for example, on the indicator when this couple is connected to the recorder both instruments will read low by 190°C. at 1,000°C. This is a very serious error. Accordingly instruments of low resistance cannot be operated alternately separately and in parallel on the same couple. They must be used always either separately or in parallel. In either case the scales of the instruments are graduated for the proper reduction in electromotive force due to line drop. It is frequently the practice to use cut out switches so that when the indicator is set on couple number 5, for example, this couple is automatically thrown out of the recorder circuit. The paralleling of instruments having a resistance of 300 ohms and more, when the line resistance is less than 3 ohms or of potentiometric instruments is a safe practice. The paralleling of instruments of lower resistance requires specially graduated scales or the use of specially wired circuits. Instruments of low resistance designed for parallel operation should not be used separately. Instruments of low resistance operated with cutout switches may be used separately since the switches are designed so that while a recorder and indicator are operated on the same line, they are never connected to the same couple at the same time.
Figure 1? illustrates a multiple thermocouple installation connected to a single indicator. Compensating lead wires are carried from the couples to a conveniently located cold-junction box. The temperature of this box is thermostatically controlled. From the cold-junction box copper wires are carried to the terminal block and selective switch illustrated. A common return has been employed between the cold-junction box and the switchboard. In general it is preferable to use individual return wires for each couple. The switchboard illustrated is designed for six couples. By pressing one of the buttons shown any desired couple is connected directly to the indicator.
For large installations several hundred couples may be connected to a switchboard and the operator of the board connects the couples successively to the indicator. The switchboard is frequently designed somewhat similar to an ordinary telephone distributing board. Often in these large installations communication between the operator of the switchboard and the operator of the furnace is maintained by a system of colored electric lamps. This method of semi-automatic temperature control is meeting with great success in the industries.
Use of Junction Box.—Figure 14-illustrates a wiring diagram for a multiple-couple installation which is very useful in saving compensating lead wire and in